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A287685
Numbers k such that 89*10^k - 9 is prime.
0
1, 7, 13, 44, 73, 128, 152, 245, 408, 436, 618, 638, 884, 1405, 1502, 1656, 2292, 2318, 4212, 5712, 5868, 13837, 18648, 25102, 30742, 34532, 53951, 159258
OFFSET
1,2
COMMENTS
For k > 0, numbers k such that the digits 88 followed by k-1 occurrences of the digit 9 followed by the digit 1 is prime (see Example section).
a(29) > 3*10^5.
EXAMPLE
7 is in this sequence because 89*10^7 - 9 = 889999991 is prime.
Initial terms and associated primes:
a(1) = 1, 881;
a(2) = 7, 889999991;
a(3) = 13, 889999999999991;
a(4) = 44, 8899999999999999999999999999999999999999999991; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[89*10^# - 9] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, May 29 2017
EXTENSIONS
a(28) from Robert Price, May 12 2020
STATUS
approved